While the use of colour to encode meaning in terms of sex/gender or devils is relatively unusual, there are other situations where colour conventions are employed in paintings. Among these are standards seen in religious paintings, such as those of the Virgin Mary.

In many paintings of the Virgin Mary, she’s shown wearing a cloak of ultramarine blue, although there are also examples of her wearing green, or a combination of red and green. In Sassoferrato’s The Virgin in Prayer, her cloak looks as if it was painted only yesterday rather than nearly four centuries ago. The choice of ultramarine may have originated from the fact that the pigment was weight-for-weight more expensive than gold, and visually even more stunning.

Dante Gabriel Rossetti’s painting of The Girlhood of Mary Virgin (1848–9) shows her embroidering with her mother Saint Anne, while her father Saint Joachim prunes a vine. Her red embroidery signifies the Passion to come, and the colour is often a symbol of Christ’s blood shed in the crucifixion. It was also adopted as the colour code for cardinals in the Roman Catholic church.

Raphael’s magnificent Portrait of a Cardinal from 1510-11 is notable not only for the lifelike modelling of flesh, but for his attention to the surface textures of the fabrics, something he had developed since his early days with Perugino. Three distinct fabrics are shown in the cardinal’s choir dress: the soft matte surface of the biretta (hat), the subtly patterned sheen of his mozzatta (cape), both in cardinal red, and the luxuriant folds of his white rochet (vestment).

With a limited range of colours available, it was inevitable that there were unfortunate conflicts, as red not only signifies the Passion and crucifixion, and cardinals, but the scarlet woman as a carnal red.

There are two origins proposed for the term scarlet woman: the earlier is the New Testament book of Revelation, in its characterisation of the Whore of Babylon in chapter 17, verses 4-5, where she’s described as being dressed in purple and scarlet, and decked with gold, precious stones and pearls. William Blake’s watercolour of The Whore of Babylon from 1809 follows this literally, although his purple has faded now.

Carolus-Duran’s Portrait of Mademoiselle de Lancey (1876) was exhibited at the Salon, and in its own way became infamous as ‘the lady with the red cushion’. Most of those who attended that Salon knew only too well that she was one of the great courtesans of the Belle Epoque, and could name many of her succession of rich lovers. The scarlets and crimsons and her direct wide-eyed gaze at the viewer left little to the imagination, and the critics were almost as merciless with Carolus-Duran as they had been in 1865 with Manet’s Olympia.

A few artists have used distinctive colour codes in other ways.

Félix Vallotton’s Andromeda Standing with Perseus (1907) shows the sea monster Cetus heading for a defenceless Andromeda, as hero Perseus charges to her aid through a cleft in the black sky. Each figure is colour coded: green for the sea monster, pink for the near-victim, and blue for the hero, against a straw-coloured sea.

Evelyn De Morgan’s Cadence of Autumn from 1905 shows five women in a frieze, against a rustic background. From the left, one holds a basket of grapes and other fruit, two are putting marrows, apples, pears and other fruit into a large net bag, held between them. The fourth crouches down from a seated position, her hands grasping leaves, and the last is stood, letting the wind blow leaves out from each hand. They wear loose robes that are coloured (from the left) lilac, gold, brown, green, and black, the sequence seen in nature.

Colour can be at its most important in multiplex narrative, to indicate each occurrence of an actor, so helping the viewer assemble each part of the story into a whole.

Masaccio’s Tribute Money (1425-8) contains three images of Saint Peter, and two of the tax gatherer, each carefully set and coherently projected into the same single view. Although each is spaced apart from the next, no pictorial device is used to separate them into frames, and they form multiplex narrative. This refers to the Gospel of Matthew, in a story in which Christ directs Peter to find a coin in the mouth of a fish so that he can pay the temple tax. In the centre, the tax collector asks Christ for the temple tax. At the far left, as indicated by Christ and Peter’s arms, Peter (for the second time) takes the coin out of the mouth of a fish. At the right, Peter (a third time) pays the tax collector (shown a second time) his due.

The use of colour to add meaning to images is longstanding practice, and can be traced back to ancient Egyptians, who tended to use it to distinguish males from females. Most probably the result of their belief system, females were often depicted with paler or even white skin, while men were more swarthy in appearance.

In this fresco of a funeral procession from the tomb of Ramose, dating from about 1353–1336 BCE, there’s a clear distinction in skin colour between the central group of women, and the men on the left and right.

This passed through into the Renaissance.

This can be most apparent in paintings of Adam and Eve, here that made by Maerten van Heemskerck in about 1550, where Eve is as white as ivory.

Annibale Carracci’s Latona and the Lycian Peasants, probably from 1590-1620, shows the near-white goddess Latona placing her curse on the swarthy-skinned locals.

After the Renaissance, this colour-coding largely disappeared, only to return at the end of the nineteenth century.

Evelyn De Morgan’s Boreas and Orithyia from about 1896 shows the darker Boreas, the north wind, bearing the paler Athenian princess Orithyia aloft.

In the early twentieth century, the former Nabi artist Félix Edouard Vallotton painted a series of narrative works, including his Perseus Killing the Dragon, from 1910, a thoroughly contemporary interpretation with a marked contrast in skin colour. This is most evident where their feet are close together at the lower edge of the painting.

Independent of that coding, paintings of hell and devils developed their own colour schemes.

This detail from Giotto’s fresco of The Last Judgment in the Scrovegni Chapel in Padua, Italy, was painted in 1306. It follows the early convention that its humanoid demons are colour-coded blue, with some in brown, in contrast to its densely-packed and near-white victims seen undergoing punishment. There’s no colour-based distinction between men and women here, though.

Luca Signorelli’s large fresco in the San Brizio Chapel in Orvieto shows the seething mass of The Damned (1499-1502). His colour-coding is richer, and there’s precious little sign of flames, fire, or even rocks, just a dense mass of people being tormented.

Bonifazio Veronese’s St Michael Vanquishing the Devil from 1530 shows a dark-skinned humanoid with draconian wings, which may have descended from older images in which the Devil (with the definite article) is shown as a straight dragon. This artist isn’t the great Paolo Veronese, but a Venetian painter whose work influenced Tintoretto.

This visual distinction extends to more recent paintings, including Henry Fuseli’s The Nightmare from 1781. The daemonic incubus seen squatting on the torso of a young woman is swarthy in colour compared to his victim’s pallor.

Ary Scheffer used clear colour coding in his Temptation of Christ, from 1854, where the fallen angel is trying to get Christ to jump from a pinnacle so that he could rely on angels to break his fall.

Most recently, some have tried to interpret this as racialism, but these codings have older origins, and only appear in skin colour, not overall appearance. It would be nonsense to suggest that any of these devils were intended to represent North Africans, for instance.